1. Field of the Invention
The present invention generally relates to a four-cycle engine, and more particularly relates to a four-cycle engine having an overhead camshaft drive.
2. Description of Related Art
Recently, outboard motors tend to use four-cycle engines for highly emission control. Such four-cycle engines typically comprise a crankshaft that drives a submerged marine propulsion device through suitable shaft couplings. Modem four-cycle engines typically employ an overhead camshaft drive system. In this system, the crankshaft also drives a camshaft(s) which actuates intake and exhaust valves. Normally, at least one flexible transmitter, such as a drive belt or chain, for example, that is disposed atop the engine, drives the camshaft(s). For instance, U.S. Pat. Nos. 5,564,380, 5,704,819, 5,848,578, 5,865,655 and 6,044,817 disclose such camshaft drives.
Some four-cycle engines can have an engine body defining first and second banks together forming a V-shape. Each bank can have intake and exhaust camshafts. The crankshaft can drive the entire intake and exhaust camshafts through a single flexible transmitter. However, the transmitter can be relatively long in this arrangement and is apt to make maintenance services troublesome. Normally, therefore, the crankshaft in another arrangement first drives the intake or exhaust camshaft in each bank through a first transmitter. Then, the intake or exhaust camshaft drives the other exhaust or intake camshaft in the same bank through a second transmitter. Three transmitters in total are used in this arrangement because two second transmitters are necessary.
Additionally, the flexible transmitter can elongate the camshaft because the transmitter needs a sprocket or pulley attached at the camshaft. Accordingly, the engine can be large to incorporate such a long camshaft.
The four-cycle engines also can be provided with a variable valve timing (VVT) mechanism to obtain high charging efficiency in a relatively high engine speed range and low fuel consumption and superior exhaust characteristics in a relatively low engine speed range. The VVT mechanism can change valve timings of either the intake or exhaust valves in response to the engine speeds. The VVT mechanism can be operated hydraulically and can include a control valve unit that controls the hydraulic operation of the VVT mechanism. The VVT mechanism often is disposed at the camshaft associated with intake or exhaust valves that need the valve timing change. The control valve unit, more specifically, controls flow of fluid in the VVT mechanism to change angular positions of the camshaft.
Due to being disposed at the portion of the camshaft, the VVT mechanism and more particularly, the control valve unit inevitably elongates the camshaft. The engine thus is larger because of the VVT mechanism and the camshaft drive, as noted above.